1. Field of the Invention
The invention relates to a bi-directional DC-DC converter having an insulating function and a method for controlling such a converter.
2. Description of the Related Arts
In recent years, hybrid vehicles having high efficiency have been being spread due to an increase in consciousness of conservation of the global environment. The hybrid automobile has a main battery for driving a motor and a battery for auxiliaries. If an electric power can be mutually supplied between the two batteries of different voltages, design flexibility of a vehicle power supply systems can be increased.
A bi-directional DC-DC converter for bi-directionally converting electric powers between two power sources of different voltages has been disclosed in JP-A-2002-165448. In the bi-directional DC-DC converter disclosed in Patent Document 1, a voltage type circuit on a high voltage side and a current type circuit on a low voltage side having a choke coil are connected through a transformer. By making a switching element of the high voltage side circuit operative, the electric power is supplied from the high voltage side power source to the low voltage side power source. By making a switching element of the low voltage side circuit operative, the electric power is supplied from the low voltage side power source to the high voltage side power source.
Further, a bi-directional DC-DC converter in which a voltage clamping circuit including an object connection in series of a switching element and a capacitor is connected to a low voltage side circuit has been disclosed in JP-A-2006-187147. In the bi-directional DC-DC converter disclosed in Patent Document 2, at the time of a step-down operation, a loss caused by a circulating current is reduced by the voltage clamping circuit. A high efficient and small bi-directional DC-DC converter in which, at the time of the step-up/down operations, the generation of a surge voltage in the low voltage side circuit is prevented and a withstanding voltage of the switching element is reduced is provided.
In the bi-directional DC-DC converter in the related art disclosed in JP-A-2002-165448, since not only the efficiency is deteriorated by a loss due to a circulating current at the time of the step-down operation but also it is necessary to raise a withstanding voltage of the switching element by surge voltages caused in the low voltage side circuit at the time of the step-up/down operations, they become an obstacle to realization of miniaturization and high efficiency.
In the bi-directional DC-DC converter in the related art disclosed in Patent Document 2 (JP-A-2006-187147), when a step-up voltage ratio/output electric power is large at the time of the step-up operation, it is also necessary to raise the withstanding voltage of the switching element in the low voltage side circuit and it becomes an obstacle to realization of miniaturization and high efficiency.